1. Field of the Invention
The present invention relates generally to digital data receivers and more specifically to a receiver for receiving digital data, particularly for optical digital signals.
2. Description of the Related Art
In a conventional de-coupled optical receiver, an optical detector delivers a current output proportional to the optical power input received by the detector. This current is converted to a voltage by a current-to-voltage converter or transimpedance amplifier, and delivered to one input of a decision circuit where an intermediate voltage is compared to a reference voltage or decision threshold and the analog input signal to a digital output signal is converted at one of discrete values depending on whether the input is above or below the reference voltage. Ideally, the dc level of the transimpedance amplifier output should match the logic threshold of the decision circuit so that the amplifier output swings symmetrically above and below the reference voltage depending on the presence or absence of an optical input. When the optical input signal is very small, the corresponding voltage swing at the amplifier output will be small and the decision circuit may be unable to detect logic transition. Moreover, even if the reference voltage is chosen such that logic transition is detectable, pulse-width distortion will result if the reference voltage is not centered at one-half of the sum of the minimum and maximum excursions of the input signal. Correspondingly, if the amplitude of the input pulse changes, the reference voltage must also change to minimize pulse-width distortion. U.S. Pat. No. 5,025,456, Y. Ota et al., discloses an adaptive circuit technique for a burst-mode optical receiver that measures the amplitude of an incoming signal and sets the proper reference level during the period of a burst. The adaptive circuit includes a differential amplifier having a first input for receiving a data input signal and a second input for receiving a reference signal. A peak detector is connected to the output of the differential amplifier via a feedback resistor for generating the reference signal such that the amplifier has a first gain value and a first bandwidth when the data input signal is less than its peak amplitude or during the absence of the data input signal and has a second gain value approximately equal to twice the first gain value and a second bandwidth approximately one-half the first bandwidth for a predetermined time after the peak amplitude of the data input signal is reached. The modes of operation in which the amplifier has the first and second gain values are called a "cold" mode and a "warm" mode, respectively.
Various problems are inherent in the prior art burst-mode optical receiver. One shortcoming is that when a dc offset occurs in the optical input due to photodetector's dark current, low extinction ratio and optical crosstalk and the like, the differential amplifier cannot adapt to the changing dc level and the reference signal remains unchanged. As a result, it is likely that a relatively low-level optical input exceeds the level of the reference signal and erroneously interpreted as a high-level signal.
Another aspect of interest is the thermal noise generated by the feedback resistor. To minimize the thermal noise it is important that the feedback resistor be chosen to have as large a resistance value as the bandwidth would permit. During the cold mode, the minimum amplitude of an optical data input signal that can be detected is determined by the difference between the reference voltage and the amplifier output that is generated in response to a series of relatively low-level data input signals. This voltage difference determines the feedback resistor for the cold mode. However, this voltage difference is be determined only in a range where a sufficient margin is allowed for the noise. On the other hand, the resistance of the feedback resistor for the warm mode can be chosen in a range which the bandwidth of the warm mode permits. However, with the prior art optical receiver the feedback resistor is limited to one half of its optimum resistance value of the warm mode if it is optimized for the cold mode.
Additionally, if the input signal is an initial sequence of relatively high amplitude pulses followed by a sequence of relatively low amplitude pulses, the output of the peak detector would decay due to spontaneous discharge during the period of the subsequent low-amplitude pulse sequence and the transimpedance amplifier switches from a "warm" mode to a "cold" mode. Therefore, the transimpedance amplifier is not quickly adaptive to varying input levels, failing to adjust its decision threshold prior to the arrival of the subsequent low-amplitude pulse sequence and resulting in decision errors. This required that successive pulse sequences be spaced at sufficient amount of time interval, or guard time.
A further aspect of interest is the limitations imposed on design freedom. Because of the necessity to simultaneously meet the requirement of a low-noise differential amplifier and the requirement of a high-speed peak detector that can operate with a sufficient phase margin to prevent voltage-follower oscillation, and because of the necessity that they have substantially matched operating characteristics, the freedom of design has been limited.